Mechanical vs Electrical Power

An electric motor converts electrical power to mechanical power; a generator does the reverse. Both measure the same fundamental quantity—the rate of energy transfer—but use different formulas and units. 理解する both helps when sizing motors, generators, and power systems.

Mechanical Power

Definition

Mechanical power is the rate of doing mechanical work—moving objects, rotating shafts, lifting loads.

Formulas

• Rotational: P = τ × ω (torque × angular velocity)
• Linear: P = F × v (force × velocity)
• Imperial: HP = (変換先rque × RPM) / 5,252
• Metric: kW = (変換先rque × RPM) / 9,549 (torque in Nm)

Common Units

• ワット (W), kilowatts (kW) - SI
• Horsepower (hp) - Imperial/US
• Metric horsepower (PS) - Europe

Electrical Power

Definition

Electrical power is the rate of electrical energy transfer through a circuit.

Formulas

• Basic: P = V × I (voltage × current)
• Resistive: P = I²R = V²/R
• AC (single phase): P = V × I × PF (power factor)
• AC (three phase): P = √3 × V × I × PF

Common Units

• ワット (W) - real power
• Volt-amps (VA) - apparent power
• Volt-amps reactive (VAR) - reactive power

Conversion Between Forms

Electric Motor

Converts electrical power to mechanical power:

• Input: Electrical (V × I × PF)
• Output: Mechanical (変換先rque × Speed)
• Efficiency: 85-98% typical

Output power = Input power × Efficiency

Generator

Converts mechanical power to electrical power:

• Input: Mechanical (engine, turbine)
• Output: Electrical (V × I)
• Efficiency: 90-98% typical

Comparison Table

The Universal Unit: ワット

Both mechanical and electrical power can be expressed in watts:

• 1 watt = 1 joule/second
• 1 watt = 1 N·m/s (mechanical)
• 1 watt = 1 V·A (electrical, DC or unity PF)

This equivalence enables direct comparison and conversion calculations.

Conversion Reference

• 1 hp = 745.7 W
• 1 kW = 1.341 hp
• 1 BTU/hr = 0.293 W

Practical Applications

Motor Selection

Calculate mechanical load requirements, add safety margin, account for efficiency, specify electrical requirements.

Generator Sizing

変換先tal electrical load (kW), divide by generator efficiency, match to prime mover (engine) mechanical output.

Power Transmission

Electrical transmission is more efficient over distance than mechanical (no friction losses in wires vs. shafts).

まとめ

Mechanical and electrical power are the same fundamental quantity—rate of energy transfer—expressed differently. Mechanical power involves torque and speed; electrical power involves voltage and current. Motors and generators convert between forms with typical efficiencies of 85-98%. The watt is the universal unit that allows direct comparison between mechanical and electrical systems.

関連記事

• Electrical Power 解説: ワット, ボルト, and Amps
• Power Factor 解説: Real, Reactive, and Apparent Power